Engineering Geology Earth Structure • Hussien aldeeky 1
Earth major spheres 1. Hydrosphere • Ocean is the most prominent feature of the hydrosphere. - Is nearly 71% of Earth's surface - Holds about 97% of Earth's water • Fresh water found in streams, lakes, and glaciers, underground water 2. Atmosphere • Thin, blanket of air • One half lies below 5.6 kilometers (3.5 miles) 3. Biosphere • Includes all life • Concentrated near the surface in a zone that extends from the ocean floor upward for several kilometers into the 2
4. Geosphere To the engineer interested in earthquake effects, the Geospere is a sphere having the layered structure of a boiled egg. It has a crust (the shell), a mantle (the egg white), and a core (the yolk.) Crust: Continental crust (25-40 km ) Oceanic crust (~6 km) Mantle Upper mantle (650 km) Lower mantle (2235 km) Core Outer core: liquid (2270 km) Inner core: solid (1216 km) Values in brackets represent the approximate thickness of each layer
Interior Structure of the Earth 4 4
Interior Structure of the Earth 5
Layers of the Earth It is important to note that there has been, so far, no drill that has penetrated the surface of the earth more than a few kilometers. Almost all information about the internal structure of the earth is inferred from observed characteristics and propagation (travel rates and reflections) of seismic waves. Magnetic and gravitational observations also help complete the picture.
Layers of the Earth The earth is divided into three main layers: Inner core, outer core, mantle and crust . The core is composed mostly of iron (Fe) and is so hot that the outer core is molten , with about 10% sulphur (S). The inner core is under such extreme pressure that it remains solid. Most of the Earth's mass is in the mantle, which is composed of iron (Fe), magnesium (Mg), aluminum (Al), silicon (Si), and oxygen (O) silicate compounds. At over 1000 degrees C, the mantle is solid but can deform slowly in a plastic manner.
THE CRUST The crust is much thinner than any of the other layers, and is composed of the least dense calcium (Ca) and sodium (Na) aluminum- silicate minerals. Being relatively cold, the crust is rocky and brittle , so it can fracture in earthquakes . The shell of the earth, the crust, can be said to have two different thicknesses. Under the oceans, it is relatively thin. It varies in thickness from 5 to 8 km. Under the land masses, it is relatively thick. The thickness of the continental crust varies from 10 to 65 km.
THE CRUST The egg shell analogy for the crust is not an exaggeration. It is paper thin compared with the radius of the earth which is approximately 6400 km. The total weight of the continental crust is less than 0.3% of the weight of the earth. Variations in the crust thickness are compensated by the weight of the water and the differences in the specific gravities of the crust under the oceans (3.0 to 3.1) and under the continents(2.7 to 2.8).
THE CRUST . Materials of the earth crust Basic knowledge of earth materials ( rocks ) is essential to the understanding of all geologic phenomena The crust is composed of two basic rock type: granite and basalt. The crust is also the source of many of the minerals and other substances that we use in industry and other fields. The continental crust is composed mostly of granite. The oceanic crust consists of volcanic lava rock called basalt. Basaltic rocks of the ocean plates are much denser and heavier than the granitic rock of the continental plates. Because of this the continents rides on the denser oceanic plates.
THE MOHO The Moho, refers to a zone or a thin shell below the crust of the earth that varies in thickness from 1 to 3 km.
THE MOHO In seismology, the term "discontinuity" is used in its general sense. It refers to a change over a short distance of a material property. In this case, the "short distance" may be as long as 3 km, a trifle compared with the radius of the earth. In that zone, the P-wave velocity has been observed to increase from approximately 6 to approximately 8 km/sec. The Moho is considered to be the boundary between the crust and the mantle. The increase in P-wave velocity is ascribed to change in composition of the medium. Rocks of the mantle are poorer in silicon but richer in iron and magnesium
THE MANTLE The mantle can be thought of having three different layers. The separation is made because of different deformational properties in the mantle inferred from seismic wave measurements. (1) The upper layer is stiff. It is presumed that if the entire mantle had been as stiff, the outer shell of the earth would have been static. This stiff layer of the mantle and the overlying crust are referred to as the lithosphere . The lithosphere is approximately 80- km thick
THE MANTLE (2) Beneath the lithosphere is a soft layer of mantle called the asthenosphere. Its thickness is inferred to be several times that of the lithosphere . One may think of this as a film of lubricant although film is not exactly the word for something so thick. It is assumed that the lithosphere, protruding (meaning: extending beyond) parts and all, can glide over the asthenosphere with little distortion of the lithosphere
THE MANTLE (3) The mesosphere is the lowest layer of the mantle. Considering the vagueness in defining the lower boundary of the asthenosphere it would be expected that the thickness and material properties of the mesosphere are not well known. It is expected to have a stiffness somewhere between those of the lithosphere and the asthenosphere.
THE CORE At a depth of approximately 2900 km, there is a large reduction (on the order of 40%) in the measured velocity of seismic waves. The boundary between the mantle and the core is assumed to be at this depth. Because no S-wave has been observed to travel through the material below this boundary for a thickness of approximately 2300 km, it has been inferred that the core comprises two layers. The 2300-km thick outer layer which is in a molten state and an 1100-km thick inner layer which is solid.
THE CORE It is known that the pressure increases toward the center of the earth. So does the temperature. The liquid outer layer versus the solid inner layer is rationalized by recognizing that the melting point of the material increases (with pressure) at a faster rate than the temperature as the center of the earth is approached .
18
Earth’s Structure Chemical Physical Structure Composition Composition Crust Oceanic Basalt: O 2 , Si, Mg, Fe Lithosphere Continental Granite: O 2 , Si, Al (cool, rigid) Mantle Uppermost Asthenosphere (hot, Asthenosphere O 2 , Fe, Mg, Si flowing) Mantle Mantle (hotter, denser) Outer (hottest, viscous Outer liquid, 4x denser than crust) Core Fe, Ni Inner (hottest, solid, 6x Inner denser than crust) 19
Stony Meteorites => Earth’s Crust Stony-Iron Meteroites => Earth’s Mantle Iron Meteorites => Earth’s Core 20
Recommend
More recommend
